It just list the states where one or both exams (theory, practical) are required (column 2 or 3 has "ja") or exempted ("nein"). If a state is missing (like California), a Cali licence holder needs to take both exams to get their licence converted to a German one.

I'm not sure if they'd have to do the whole driving school (there are mandatory 12 hours one has to drive in Germany, with a driving teacher), or if they can just pass the exams and get a German licence.

Interestingly, Switzerland converts all US licences with no exam requirements. I wonder if someone can move there, get their licence converted, and then move to Germany and convert their Swiss licence to a German one...

It's generally due to reciprocity, so states that allow exchanging German licenses will be much more likely to be on the list, and vice versa.